Electrophotographic processes and apparatus employ the use of toners, which are generally comprised of a resin and a colorant, along with other desirable additives like charge control agents. In general, a desired image is formed on an organic photoconductor (OPC) coated medium such as a drum or belt in the form of a charged pattern representing the image. Toner is then electrically attracted to the charge on the drum and adheres to the drum in an imagewise manner. Lastly, the toner image is transferred from the OPC medium to an image-receiving substrate (typically paper) and fused, resulting in permanent image formation on the substrate.
In magnetic brush development systems, charge is imparted to the toner triboelectrically by mixing toner particles with carrier particles, which are typically resin-coated steel particles about 20 to 200.mu. in diameter. The toner particles adhere to the oppositely-charged carrier particles and are conveyed from a hopper to the magnetic brush roller system. On the roller chains of the toner-laden carrier particles form, and as the chains are conveyed on the roller into the gap between the roller and the OPC medium, the charged toner particles are attracted to and deposited on the oppositely-charged latent image areas of the OPC medium. The carrier particles are collected and recycled for remixing with toner.
Since the carrier is a recyclable component of the developer, it is naturally of great concern to make it last as long as possible so as to minimize cost. A persistent problem in the art is "toner scum", wherein after a period of mixing with toner, toner particles irreversibly adhere to the carrier, rendering triboelectric charging ineffective and necessitating replacement of the carrier.
A related problem is the selection of coatings for the carrier particles. When designing developer for copying machines, many factors must be considered, one of which is the triboelectric characteristics of the carrier particles vis-a-vis the particular toner required by the copier mechanics. Much development work is devoted to toner chemistry and physics and the goal of having the toner form a good printed image on the paper. However, because the toner cannot form an image on the photoconductor unless the toner is sufficiently triboelectrically charged, then subsequently separated from the carrier particles during imaging (within the mechanical and physical copier design parameters chosen), the chemical and physical design of the carrier must be considered also. Therefore, carrier and toner must be engineered to work together as a developer. When many different developer formulations are under development, for many different machines, this can be a complicated effort. It would therefore be preferable to be able to formulate and characterize one carrier formulation that can be easily and predictably optimized to the particular demands of any given toner and its respective copying system, so as to produce a "tailor-made" developer at a lower cost.
Another issue that arises in toner manufacture is that of unremoved solvent such as methyl ethyl ketone (MEK) in the carrier coating after the carrier has been processed. Trace amounts of solvent in the carrier coating are deleterious to developer performance because toner can adhere to carrier particles having unremoved MEK. Furthermore, the MEK will dissolve the toner, creating a sticky residue on the surface of the carrier that greatly reduces the triboelectric effectiveness of the carrier.